This proposal is concerned with investigations of molecular interactions which are involved in the regulation of muscular contraction. Solution conformational dynamics of actin and troponin subunits will be studied by static and time-resolved fluorescence spectroscopy, using both labeled and unlabeled proteins. The interactions of the proteins with each other and with tropomyosin will also be studied. From these investigations and studies of the effect of myosin binding on the conformation of unregulated and regulated thin filaments information about conformational changes produced by these interactions will be obtained. Transfer of excitation energy will be used to study proximity relationship within the thin filaments. The transient kinetics of Ca ion binding to troponin C, the solution dynamics of the Ca ion-troponin C complex, and the effect of interactions with other proteins on these properties will be investigated by stopped-flow and pulsed nanosecond fluorimetry. From these studies information about transient conformations and changes of these conformations and about the regulation of contractile activity by these processes will be obtained. The fluidity of sarcoplasmic reticulum vesicles will be studied by analysis of the time course of both monomer and excimer emission of incorporated pyrene. These results are expected to enhance our general knowledge of contractility and take us one step closer toward the elucidation of the regulatory mechanism of muscle contraction.